Method for producing a read-only memory having a plurality of MISFET

ABSTRACT

A read-only memory device includes a number of MIS transistors forming memory cells arranged in a matrix configuration to provide a NOR type memory device with high current driving capability for the memory cells. Bit lines and column lines are arrayed alternately in common in each cell column so as to be used in common by adjacent memory cells in the word line extending direction. The bit lines for reading out signals from the memory cells function as the sources or drains of the MIS transistors of the memory cells, whereas the column lines for supplying the constant voltage to the memory cells function as the drains or sources of the MIS transistors of the memory cells. For column selection, there is provided a first selection switch for selecting a group consisting of a plurality of bit lines and a plurality of column lines. A second selection switch and a third selection switch are provided for selecting the bit line and the column line of the group, respectively. The second and the third selection switches may be arranged with a layout allowance and, if these second and third selection switches are formed by MIS transistors similar to those of the memory cells, the direction in common with the memory cells may be the channel direction to contribute to improved circuit integration. The MIS transistor constituting the memory cell may be of such a construction in which the source and drain regions may be provided below the thick insulating film formed on the substrate surface, such as field oxide film.

This is a division, of application Ser. No. 603,931, filed Oct. 26,1990, now U.S. Pat. No. 5,202,848

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a read-only memory device adapted for readingout data programmed in a memory cell and, more particularly, to aread-only memory device having NOR type memory cells.

2. Description of the Prior Art

With the read-only memory device (ROMs) adapted for storing a largeamount of data and reading out the stored data when necessary, a demandfor a higher degree of integration has been raised in keeping pace withpropagation of OA equipment or electric computers.

As a typical structure for realizing such high integration, there isknown a mask ROM having a NAND type cell circuit construction which hasa so-called multi-gate structure provided with a gate electrode layerformed by two polysilicon layers, and a shallow groove or trench, asdisclosed for example in "8M and 16M mask ROM Employing Shallow Trench"in a monthly magazine entitled "Semiconductor World", October, 1987,pages 33 to 38. On the other hand, there is known a mask ROM having aNOR type cell circuit construction which has its source-drain regionconstructed by a diffusion region, as disclosed for example in "16 MbROM Design Using Bank Select Architecture" in 1988 Symposium on VLSLcircuits, Japan Association of Applied Physics, material, VL-7, pages 85to 86.

FIG. 1 is a circuit diagram showing essential portions of the mask ROMhaving the NOR type memory cells. With this mask ROM, each of the memorycells 200 arranged in a matrix configuration and adapted to be selectedby word lines W₁ to W₈ is constituted by each one MOS transistor, thesource-drain region of which is formed by a diffusion region whichproves to be common bit lines 205, 206 and 207 for each column. Withthis mask ROM, virtual grounding lines 201 and main bit lines 202 areformed alternately, with the direction normal to the extending directionof the word lines as the longitudinal direction. These virtual groundinglines 201 and the main bit lines 202 are wired with a shift of one bitline one so as to be connected to different columns for one and theother of memory cell blocks. Thus the same bit line may be connected tothe virtual grounding line 201 or to the main bit line 202 byalternative selection or bank selection of selection transistors 203 and204. For read-out, a given column is selected by column selectiontransistor 208 and a word line is selected so that data may be read outby means of the main bit line 202 and sense amplifier 209 andtransmitted to an output terminal.

In FIG. 2, which shows the layout of the mask ROM shown in FIG. 1, theregion indicated by numerous dots indicates polysilicon layers which arearrayed parallel to one another with the X-direction as the longitudinaldirection. The region sandwitched between a pair of contact holes 211,211 in the Y-direction represents a memory block. A diffusion region 213where the contact hole 211 is formed is roughly of a pattern in the formof a letter H. Diffusion regions 205 to 207, . . . indicate bit linesfunctioning as the source-drain region. The channel of the transistor ofeach cell is formed in the lower portions of the word lines W₁ to W₈.Programming in performed by introducing impurities into the channel withthe use of a mask pattern 212. Gate electrodes SO_(i), SO_(i+1) of aselection transistor 204 and gate electrodes SE_(i), SE_(i+1) of aselection transistor 203 used for bank selection are extended parallelto the word lines W₁ to W₈, and are arranged on both sides of the memoryblock.

With the above described NOR type cell, the memory cell drivingcapability is lowered when the number of serially connected transistorsis increased with a view to achieving a higher degree of integration.

On the other hand, with the NOR type cell mask ROM, the following layoutproblems are raised.

The transistor channel of each cell is formed at the lower portion ofthe word lines W₁ to W₈, with the channel direction being theX-direction. However, with the selection transistors 203, 204 for bankselection, the channel region is formed between the bit lines 205 to207, . . . and the substantially H-shaped diffusion region 213, despitethe fact that the gate electrodes SE_(i), SE_(i-1), SO_(i) and SO_(i+1)thereof run parallel to word lines W₁ to W₈. Thus the channel directionof these selection transistors is the Y-direction. Therefore, with theselection transistors 203, 204 for bank selection, the channel stopregion needs to be formed between the adjoining channels in the lowerportions of the gate electrodes SE_(i), SE_(i-1), SO_(i) and SO_(i+1).Thus, while it suffices in the memory cell region to perform channelstop ion implantation in alignment with word lines W₁ to W₈, it isnecessary to perform ion implantation separately from that for thememory cell region. Since the channel stop region cannot be formed inalignment with the polysilicon layer, it is necessary to provide amargin to take account of mask deviation. The necessity for providingsuch a margin results in limitations on the memory block side region andconsequent difficulties in improving the degree of integration of thememory device.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a read-only memorydevice in which the memory cell driving capability may be enhanceddespite the elevated degree of integration and in which a high degree ofintegration may be achieved from the viewpoint of layout.

According to the read-only memory device of the present invention, aplurality of cells each including a MIS transistor having a differentthreshold voltage in accordance with the programmed data are arranged ina matrix configuration to form NOR type cells. The memory deviceincludes word lines functioning as the gates of the MIS transistors andused in common in the cell rows and bit and column lines used as sets inthe cell columns and used in common in the cells adjacent to each otherin the extending direction of the word lines. The bit lines and thecolumn lines are arrayed alternately in the extending direction of theword lines. Thus, one of the sources or the drains of the MIStransistors arrayed in each cell column is used as the bit line for dataread-out while the other of the sources or the drains of the MIStransistors arrayed in each cell column is used as the column line towhich a predetermined potential is applied. A plurality of the bit linesand a plurality of the column lines make up a group, and a plurality ofsuch groups are provided in the memory device in its entirety. There areprovided first selection means for selecting one of the groups, secondselection means for selecting the bit line of the ground and thirdselection means for selecting the column line of the group.

Since the read-only memory device of the present invention is providedwith the NOR type cells, the memory cell driving capability is improved.The bit lines and the column lines may be connected fixedly as a resultof the above described arraying of the bit lines and the column linesand the provision of the first to third selection means, so that thesecond and the third selection means may be arrayed with a sufficientlayout margin.

In one aspect of the present invention, MIS transistors are used as thesecond and the third selection means. With the channel direction ofthese MIS transistors being the same as the channel direction of the MIStransistors of the cells, a higher degree of integration may be achievedby the aligned formation of the selection means similar to that of thememory cells.

In another aspect, the second and the third selection means are formedin a staggered relation with respect to the memory cells forfacilitating the wiring for layout. If necessary, the memory device maybe divided into plural memory blocks and, in addition, a load circuitmay be provided in the bit or column lines.

With the above described read-only memory device, NOR type memory cellsare formed on the semiconductor substrate. With a memory deviceaccording to a modification of the present invention an impurity regionhaving an opposite conductivity type to that of the semiconductorsubstrate is formed below a plurality of parallel thick insulating filmsformed in striped pattern on the semiconductor substrate. A plurality ofelectrode layers extending substantially orthogonally to the stripedpattern of the insulating films are formed on thin insulating filmsbetween the thick striped insulating films. Impurities are selectivelyintroduced into the substrate region below the electrode layersfunctioning as the gates of the MIS transistors. The electrode layersmay be either of a single-layer structure or a double-layer structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing essential portion of an example ofthe conventional read-only memory device.

FIG. 2 is a plan view showing the layout thereof.

FIG. 3 is a circuit diagram showing essential portions of an embodimentof the read-only memory device of the present invention.

FIG. 4 is a timing chart for illustrating the operation of the memorydevice shown in FIG. 3.

FIG. 5 is a block diagram of the memory device shown in FIG. 3.

FIG. 6 is a block diagram showing the memory device of FIG. 3 whendivided into a plurality of memory blocks.

FIG. 7 is a plan view showing the layout of the memory device shown inFIG. 3, wherein the electrode layer is formed by a single-layerpolysilicon layer.

FIG. 8 is a plan view similar to FIG. 7 wherein the electrode layer isformed by a double-layer polysilicon layer.

FIG. 9 is a plan view showing the memory cell layout of the memorydevice shown in FIG. 3.

FIG. 10 is a cross-sectional view taken along line X--X of FIG. 9.

FIG. 11 is a cross-sectional view taken along line XI--XI of FIG. 9.

FIG. 12 is a cross-sectional view taken along line XII--XII of FIG. 9.

FIG. 13 is a cross-sectional view taken along line XIII--XIII of FIG. 9.

FIGS. 14a to 14c illustrate the steps for forming the source-drainregion of the memory transistor of the memory device shown in FIG. 3.

FIGS. 15a to 15c illustrate the steps for forming the electrode layersof the memory transistor.

FIG. 16 is a circuit diagram showing essential portions of amodification of the read-only device according to the present invention.

FIG. 17 is a circuit diagram showing essential portions of a furthermodification of the read-only memory device according to the presentinvention.

FIG. 18 is a timing chart for illustrating the operation of the memorydevice shown in FIG. 17.

FIG. 19 is a plan view showing the layout of a column selection circuitof the memory device shown in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

The present embodiment is an example of a read-only memory (ROM) devicehaving memory cells arrayed in a matrix, wherein source-drain regions ofMOS transistors of the memory cells are bit and column lines which arearranged alternately. Impurities are selectively introduced by ionimplantation into channel regions of the MOS transistors of the memorycells for programming the transistors.

Overall General Arrangement (FIG. 5)

The ROM of the present embodiment has an overall general construction asshown in FIG. 5. Thus the ROM has a cell array 4 made up of a largenumber of memory cells in a matrix configuration and a row decoder 6 anda column decoder 7 for selecting the rows and columns in the cell array4, respectively. Signals are transmitted via row decoder 6 and columndecoder 7 from an address buffer 5. These signals from the addressbuffer 5 are derived from address signals A_(x) from outside. Date fromthe cell array 4 are amplified via column decoder 7 by a sense amplifier8 so as to be transmitted to an output buffer 9. An output bufferD_(OUT) is taken out to outside from output buffer 9.

Cell Array Arrangement (FIG. 3)

The arrangement of a cell array section of the ROM of the presentembodiment is explained by referring to FIG. 3. Meanwhile, FIG. 3 showsonly a portion of a unit structure which is repeated in the extendingdirection of the word lines. The number of the word lines, which in FIG.3 is four for convenience sake, is eight or more, as will be explainedsubsequently.

In each memory cell block 1, memory cells are arrayed in a matrixconfiguration. Each memory cell is constituted by a n-channel MOStransistor. The threshold value of each MOS transistor is selectivelyadjusted or programmed to a high threshold voltage or a low thresholdvoltage as a function of the programmed data. The gate electrodes ofthese MOS transistors are word lines W₁ to W₄ which are extended withthe horizontal direction in the drawing as the longitudinal direction,and are shared in common by the MOS transistors of each row. One of thesource or drain regions of the MOS transistor of each cell is the bitline B₁₂, B₂₁, B₂₂ or B₃₁, while the other of the source or drainregions is the column line C₁₁, C₁₂, C₂₁ or C₂₂. These bit and columnlines are extended with the direction normal to the word lines W₁ to W₄as the longitudinal direction. These bit and column lines are shared incommon by the MOS transistors lying along the extension of the wordlines. Thus the bit lines B₁₂, B₂₁, B₂₂ and B₃₁ and the column linesC₁₁, C₁₂, C₂₁ and C₂₂ are arrayed alternately along the line ofextension of the word lines in such a manner that the bit line B₁₂ isfollowed by the column line C₁₁, which is the followed by bit B₂₁, andso forth, with the column lines and the word lines being arrayedalternately.

At one end of the memory cell block 1, there are provided MOStransistors T₁, T₂, T₃ and T₄ as second selection means foralternatively connecting the bit lines B₁₂, B₂₁, B₂₂ and B₃₁ to the mainbit lines B₁, B₂ and B₃. That is, the bit line B₁₂ is connected via MOStransistor T₁ to the main bit line B₁, the bit line B₂₁ is connected viaMOS transistor T₂ to the main bit line B₂, the bit line B₂₂ is connectedvia MOS transistor T₃ to the main bit line B₂ and the bit line B₃₁ isconnected via MOS transistor T₄ to the bit line B₃. Thus, each main bitline is electrically connected to one of paired bit lines by the MOStransistor as the second selection means. Each of the MOS transistors T₁and T₃ has its gate electrode as a selection line WBS, while each of theMOS transistors T₂ and T₄ has its gate electrode as a selection lineWBS. The signals supplied to the selection line WBS and the selectionline WBS are in antiphase to each other. Thus, when the selection lineWBS is at a high level, the main bit line B₂, for example, is connectedvia MOS transistor T₂ to bit line B₂₁. Conversely, when the selectionline WBS is at a high level, the same main bit line B₂ is connected viaMOS transistor T₃ to bit line B₂₂. The operation for the other main bitlines is similar to that described above.

At the other end of the memory cell block 1, there are provided MOStransistors T₅, T₆, T₇, T₈ and T₉ as third selection means. These MOStransistors T₅, T₆, T₇, T₈ and T₉ are used as switches for electricallyconnecting the main column lines C₁ and C₂ to column lines C₁₁, C₁₂, C₂₁and C₂₂. Thus the main column line C₁ is connected to column line C₁₁ bymeans of MOS transistor T₆, while being connected to column line C₁₂ bymeans of MOS transistor T₇. The main column line C₂ is connected tocolumn line C₂₁ by means of MOS transistor T₈, while being connected tocolumn line C₂₂ by means of MOS transistor T₉. Similar electricalconnections are made for the remaining main column lines. Of the MOStransistors, functioning as the third selection means, each of the MOStransistors T₆ and T₈ has its gate electrode as selection line WCS,while each of the MOS transistors T₅, T₇ and T₉ has its gate electrodeas selection line WCS. The signal supplied to selection line WCS is inantiphase with respect to the signal supplied to selection line WCS.Thus, when the selection line WCS is at a high level, MOS transistors T₆and T₈ are turned on, so that the main column line C₁ is electricallyconnected to the column line C₁₁, at the same time that the main columnline C₂ is electrically connected to the column line C₂₁. Conversely,when the selection line WCS is at the high level, MOS transistors T₇ andT₉ are turned on, so that the main column line C₁ is electricallyconnected to the column line C₁₂, at the same time that the main columnline C₂ is electrically connected to the main column line C₂₂.

The main bit lines B₁, B₂ and B₃ and the main column lines C₁ and C₂,alternatively connected to the bit lines or to the column lines as afunction of signals supplied to the selection lines, are extended acrossthe memory cell block 1 in a direction orthogonal to the word linedirection. To one ends of the main bit lines B₁, B₂ and B₃ and the maincolumn lines C₁ and C₂ is connected a load circuit 3 constructed by loadtransistors T₁₈ to T₂₂. These load transistors T₁₈ to T₂₂ are connectedto the main bit lines B₁ to B₃ and to the main column lines C₁ and C₂,respectively. A source voltage Vcc is applied to these main bit lines B₁to B₃ and to main column lines C₁ and C₂ by way of the associated loadtransistors T₁₈, T₁₉, T₂₀, T₂₁ and T₂₂. The gate electrodes of the loadtransistors T₁₈, T₁₉, T₂₀, T₂₁ and T₂₂ are connected in common and aresupplied with an impedance controlling signal φ₁.

On the side of the memory cell block 1 opposite to the load circuit 3,there is provided a column selection circuit 2 as first selection means.This column selection circuit 2 selects a given row of the memory cellblock on the group-by-group basis in dependence upon signals Y₁ and Y₂from the column decoder. That is, while the main column line is selectedby the signals Y₁ and Y₂, it is the operation of the MOS transistors T₅,T₆, T₇, T₈ or T₉ as third selection means which decides to which columnline the selected main column line is to be connected. Also, while themain bit line of the two main bit lines according to the presentembodiment is selected by the signal Y₁ and Y₂, it is the operation ofthe MOS transistors T₁ to T₄ as second selection means which decides towhich column line the selected main bit line is to be connected. Withthe column selection circuit 2 of the present embodiment, the maincolumn line is mainly selected by the signals Y₁ and Y₂, at the sametime that the main bit lines associated with the main column lines areselected. However, the main bit line may also be selected by the signalsY₁ and Y₂. With the present embodiment, each group is constituted by twocolumn lines and three bit lines. When assumed that only the signal Y₁is set to a high level, so that the column associated with the signalY₁, is selected, while the second and third selection means are as yetnot in operation, the cells or columns that may possibly be selected arethe cells on both sides of the column lines C₁₁ and C₁₂, such that onlythe cells within the group may be read out. Alternative selection of thebit and column lines within the group is performed by the second andthird selection means.

A more specific construction of the column selection circuit 2 ishereinafter explained. The main bit line B₁ is connected to a data busline by means of a MOS transistor T₁₀ and another MOS transistor, notshown. The main bit line B₂ is connected to the data bus line by meansof MOS transistors T₁₃ and T₁₄, while the main bit line B₃ is connectedto the data bus line by means of the MOS transistor T₁₇ and another MOStransistor, not shown. The main column line C₁ is connected to agrounding line by means of MOS transistors T₁₁ and T₁₂, while the maincolumn line C₂ is connected to a grounding line by means of MOStransistors T₁₅ and T₁₆. The gates of the MOS transistors T₁₀ and T₁₁are connected to an output terminal of an AND circuit 21, the gates ofthe MOS transistors T₁₂ and T₁₃ are connected to an output terminal ofan AND circuit 22, the gates of the MOS transistors T₁₄ and T₁₅ areconnected to an output terminal of an AND circuit 23 and the gates ofthe MOS transistors T₁₆ and T₁₇ are connected to an output terminal ofan AND circuit 24. These AND circuits 21 to 24 are two-input gates toone of which the signals Y₁ or Y₂ are entered. To AND circuit 21 areentered the signal Y₁ and a logical product of the signals on selectionlines WBS and WCS. To the AND circuit 22 are entered the signal Y₁ and alogical product of the signals on selection lines WBS and WCS. To theAND circuit 23 are entered the signal Y₂ and a logical product of thesignals on selection lines WBS and WCS. To the AND circuit 24 areentered the signal Y₂ and a logical sum of the signals on selectionlines WBS and WCS. Thus it is only when both the signal on the selectionlines WBS and WCS are at high level that the outputs of the AND circuits21, 23 are at a high level and otherwise it is the outputs of the ANDcircuits 22, 24 that are at a high level.

Description of the Read-out Operation (FIG. 3 and 4)

The read-out operation of a read-only memory of the present embodimentshown in FIG. 3 will be explained by referring to FIG. 4.

Referring to FIG. 4, the signal φ₁ is initially changed from the low (L)level to the high (H) level, the impedances of the load transistors T₁₈,T₁₉, T₂₀, T₂₁ and T₂₂ of the load circuit 3 are at predetermined levelsand the main bit line B₁, B₂ and B₃ and the main column lines C₁ and C₂in the non-selected state are raised to the side of the source voltageVcc.

The operation will be explained for the case in which the memorytransistors M₁ to M₇ of the first row are read out sequentially. Thepotential on the word line W₁ is raised from the "L" level to the "H"level. This selects the word line W₁ of the first row. The potential ofthe remaining word lines W₂ to W₄ is at or changed to the "L" level,thus being set to the non-selected state. On the other hand, only thesignal Y₁ is raised from the "L" level to the "H" level, responsive tothe signal from the column decoder, the remaining signal, such as thesignal Y₂, remaining at the "L" level. Thus, only the AND circuits 21and 22, supplied with the signal Y₁, are enabled, while the remainingcircuits, such as the remaining AND circuits 23 and 24, are inactivated.With the rising of the signal Y₁, signals are supplied on the selectionlines WBS and WCS for activating the second and third selection means.The selection line WBS in first set to "L" level, while the selectionline WCS is also set to "L" level. Thus the selection lines WBS, WCS areboth set to "H" level. As a result, only the output of the AND circuit21, to which the logical product of the selection line WBS and WCS isentered, is at "H" level, whereas the outputs of the remaining ANDcircuits 22 to 24 are at "L" level.

With the AND circuit 21 being at "H" level, the MOS transistors T₁₀ andT₁₁ are turned on, while the MOS transistors T₁₂ to T₁₇, driven by theremaining AND circuits 21 to 24, remain in the off state. In thismanner, MOS transistors T₁₀ and T₁₁ are turned on, with the main bitline B₁ being electrically connected to the data bus line by means ofMOS transistor T₁₀. Simultaneously, the main column C₁ is electricallyconnected to the grounding line by means of the MOS transistor T₁₁ forfunctioning as the virtual grounding line. With the main column line c₁being thus connected to the grounding line, the potential at the maincolumn line C₁ is lowered.

Simultaneously, since the selection lines WBS and WCS are both at the"H" level, the MOS transistors T₁ and T₃ of the second selection meansare turned on and the MOS transistors T₆ and T₈ as the third selectionmeans are turned on. The remaining MOS transistors T₂, T₄, T₅, T₇ and T₉of the second and third selection means remain in the off state. Sinceonly the main bit line B₁ and the main column C₁ are activated, asdescribed above, the main bit line B₁ is alternatively connected to thebit line b₁₂ by means of the MOS transistor T₁, while the main columnline C₁ is alternatively connected to the column line C₁₁ by means ofMOS transistor T₆. Of the word lines, only the word line W₁ is in the onstate. Hence, only the memory transistor M₁ has been selected at thisstage.

When the selected memory transistor M₁ has a high threshold voltage byprogramming by selective impurity ion implantation, memory transistor M₁is not turned on, without the potential at the bit line B₁₂ not beinglowered. When the memory transistor M₁ has a low threshold voltage, thepotential at the word line W₁ is turned on from the potential word lineW₁, with the potential at the main bit line B₁ being lowered. As aresult, the potential at the main bit line B₁ is lowered is lowered,with the potential at the data bus line being also lowered. Hence, thepotential at the data bus line is changed by the threshold voltage ofthe memory transistor M₁, and the output signal D_(out) may be obtainedby detecting and amplifying the potential change on the data bus line bya sense amplifier.

After the data of the transistor M₁ is read out, the selection line WBSis changed from "L" level to "H" level. The output of the AND circuit 21selected to the signal Y₁ goes low and, conversely, the output of theAND circuit 22 goes high. As a result, MOS transistor T₁₀ is turned offand the main bit line B₁ is electrically isolated from data bus line.The main column line C₁ is electrically connected to the grounding lineby means of the MOS transistor T₁₂ instead of being connected via MOStransistor T₁₁ to the grounding line. The MOS transistor T₁₃ is turnedon and the main bit line b₂ is now connected electrically to the databus line by means of MOS transistor T₁₃. Since the selection line WCSremains at the "L" level, the main column line C₁ is connected to columnline C₁₁ by means of MOS transistor T₆. Since the selection line WBS isswitched from "L" level to "H" level, MOS transistor T₂ is turned on,while MOS transistor T₃ remains off. Thus the main bit line B₂ isalternatively connected to bit line B₂₁ by means of MOS transistor T₂.With the bit line B₂₁ and the column line C₁₁ selected in this manner,the memory transistor M₂ of the row pertaining to the same word line W₁has been selected. The bit line B₂₁ is changed in accordance withprogrammed data similarly to the memory transistor M₁ and such change isdemonstrated on the data bus line by way of main bit line B₂.

During the next cycle, in order to selecting the memory transistor M₃,the potential at the selection line WCS is switched from "L" level to"H" level. The MOS transistor T₆ connected to the main bit line C₁ isturned off, while the MOS transistor T₇ is turned on, with the output ofthe AND circuit 22 remaining at the "H" level. As a result, the columnline connected electrically to the main column line C₁ is changed fromcolumn line C₁₁ to column line C₁₂, thus the memory transistor M₃ beingnow selected. The bit line B₂₁ is changed in accordance with programmeddata similarly to the memory transistor M₁ and such change isdemonstrated on the data bus line by way of main bit line B₂.

During the next cycle, the potential at the selection line WBS islowered from "H" level to "L" level. As a result, MOS transistor T₂ isturned off, while MOS transistor T₃ is turned on. The bit line connectedelectrically to main bit line B₂ is switched to bit line B₂₂. The maincolumn line C₁ is already grounded and electrically connected by MOStransistor T₇ to column line C₁₂, so that memory transistor M₄sandwitched between the column line c₁₂ and the bit line B₂₂ is elected.Similarly, data are read into data bus line by way of main bit line B₂.

After reading out the data of memory transistor M₄, signal Y₁ rises,while signal Y₂ falls, as shown in FIG. 4 so that the column selectioncircuit 2 as the first selection means has selected the next group. Withthe potentials at the selection lines WBS and WCS both set to "L" level,the potentials at signal lines WBS and WCS are at "H" level, so that theoutput of and AND circuit 23 is at "H" level. At this time, the outputsof the remaining AND circuits 21, 22 and 24 are at "L" level. With theoutput of the AND circuit 23 at the "H" level, MOS transistors T₁₄ andT₁₅ are turned on, as a result of which the main bit line B₂ and themain column line C₂ have been selected. Since the potential at thesignal line WBS is at "H" level, MOS transistor T₃ is also selected,such that the main signal line B₂ is connected to bit line B₂₂ by way ofMOS transistor T₃. On the other hand, since the potential at signal lineWCS is at "H" level, MOS transistor T₈ is turned on, such that columnline C₂₁ is electrically connected to main column line C₂. With thecolumn line C₂₁ and the bit line B₂₂ being in use, memory transistor M₅has been selected.

Then, with only the signal Y₂ remaining at "H" level, the potentials atthe selection lines WBS and WCS are changed sequentially, in the sameway as when the signal Y₁ is at "H" level, so that the memorytransistors M₆, M₇ and so forth are selected sequentially. Aftertermination of the cycle of the signal Y₂, the group-selecting signalsY₃, Y₄, . . . proceed sequentially. When the last column is reached, thepotential at word line W₁ is lowered, while the potential at word lineW₂ is raised. The memory transistors are selected similarly sequentiallyto read out data into the data bus line.

Divided Memory Construction (FIG. 6)

The ROM of the present embodiment may be divided into memory cell blocksas shown in FIG. 6. Although eight word lines are herein employed, thereis not basic difference from the ROM of FIG. 3.

The ROM shown in FIG. 6 has an n number of cell blocks MB₁, MB₂, . . . ,MB_(n) in a direction normal to the extending direction of the wordlines. Each cell block MB₁, MB₂, . . . , MB_(n) has a number of linesemployed permanently as alternate column lines and bit lines, and anumber of memory transistors arrayed in a matrix configuration. Thusthere are provided MOS transistors as second control means, which arecontrolled by selection lines WBS·X₁ to WBS·X_(n), WBS·X₁ to WBS·X_(n),and MOS transistors, as third selection means, which are controlled byselection lines WCS·X₁ to WCS·X_(n), WCS·X₁ to WCS·X_(n). With thememory device thus divided into memory cell blocks, the column and bitlines, not shown, in the cell blocks MB₁, MB₂, . . . , MB_(n), areshortened in length in a direction normal to the extending direction ofthe word lines. This reduces the values of the resistances or parasiticcapacitances to enable a high speed operation, while providing anadvantage when the bit lines and the column lines are formed asdiffusion regions.

With the above described n number of the cell blocks MB₁, MB₂, . . . ,MB_(n), bit lines B_(O) to B_(m) are provided in common and are formedin the same direction as that in which the bit lines are formed. Withthe n number of the cell blocks MB₁, MB₂, . . . MB_(n), the main columnlines C₁ to C_(m) are provided and arrayed parallel to the main bitline. The main bit lines B_(O) to B_(m) and the main column lines C₁ toC_(m) are arrayed alternately in the extending direction of the wordlines.

A load circuit 30 is provided at the end of the cell block MB₁ in thedirection normal to the extending direction of the cell block MB₁. Tothis load circuit 30 is supplied a signal φ₁ for controlling theimpedance of the MOS transistor constituting the load circuit 30.Besides this signal φ₁, a signal φ₂ may also be provided, as in anembodiment which will be explained subsequently. With the main bit linesB_(O) to B_(m) and the main column lines C₁ to C_(m) thus used in commonin each of the cell blocks MB₁, MB₂, . . . , MB_(n), the load circuit 30may be arranged at the end of the cell blocks in their entirety forrealizing the reduced area occupied by memory cells and the high degreeof integration of the memory device.

A column selection circuit 20 is provided at the end of the cell blockMB_(n) in a direction normal to the extending direction of the wordlines. Control signals WBS, WCS as well as column selection signals Y₁to Y_(m) from column decoder are supplied to the column selectioncircuit 20. By these signals, each one main column line and main bitline are selected as a group to perform the above described read-outoperation. With the main bit lines B_(O) to B_(m) and the main columnlines C₁ to C_(m) thus used in common in the cell blocks MB₁, MB₂, . . ., MB_(n), the column selection circuit 20 may be arrayed at the terminalend of the cell blocks in their entirety, as in the case of the loadcircuit 30, for realizing reduction in the area occupied by memory cellsand the high degree of integration of the memory device.

Layout for the Case the Electrode Layer is a single Polysilicon Layer(FIG. 7)

Then, referring to FIG. 7, the layout for the case in which theelectrode layer is formed as a single polysilicon layer is explained.Meanwhile, the layout of FIG. 5 is a partial view for simplifying thedescription. In practice, the layout is a continuous repetition of thepattern shown therein in the X- and Y-directions.

As shown in FIG. 7, a plurality of polysilicon layers shown by dottedareas are formed on the silicon substrate 41 for extending in theX-direction. In this layout, the area between a pair of contact holes42, 42 arrayed in the Y-direction represents one cell unit within whichselection lines WBS·X_(n) and WCS·X_(n), eight word lines W₁ to W₈ andselection lines WCS·X_(n) and WCS·X_(n) are formed as bands or stripeseach formed by a polysilicon layer. These lines are spaced apart fromeach other by a predetermined distance and ion implantation is performedin a self-aligned manner to form channel stop regions.

The patterns of bit lines B₁₁, B₁₂, B₂₁ and B₂₂ and column lines C₀₁,C₀₂, C₁₁ and C₁₂ are formed with the Y-direction as the longitudinaldirection, as shown by solid lines in the drawings. These bit lines B₁₁,B₁₂, B₂₁ and B₂₂ and the column lines C₀₁, C₀₂, C₁₁ and C₁₂ are of thestriped patterns and are constituted by impurity diffusion regionsformed at the bottom of the thick oxide film (LOCOS). The thick oxidefilm is not shown in the drawing. These bit lines B₁₁, B₁₂, B₂₁ and B₂₂and the column lines C₀₁, C₀₂, C₁₁ and C₁₂ are formed by impuritydiffusion regions formed at the lower side of the thick oxide film onthe surface of the silicon substrate 41 and hence are used as the sourceand drain regions of the memory transistors. These bit lines B₁₁, B₁₂,B₂₁ and B₂₂ and the column lines C₀₁, C₀₂, C₁₁ and C₁₂ havecharacteristic terminal positions in the cell block in order that theMOS transistors and the memory transistors functioning as the second andthird selection means may be in the same channel direction.

That is, the column lines C₀₁, C₁₁ are formed from the lower end of theword line W₁ as far as the lower end of the selection line WCS·X_(n),while the end 47 of the third selection means does not reach the lowerend of the selection line WCS·X_(n). Thus the perimeter of the contacthole 42 may be extended to an extended position on the column lines C₀₁and C₁₁. The column lines C₀₂ and C₁₂ are formed in a range extendingfrom the lower end of the word line W₁ to the lower end of the selectionline WCS·X_(n). The impurity diffusion region extending from contacthole 42 with the main column lines C₀ and C₁ is extended in theY-direction on a line of extension of the bit line and has a terminalpoint 48 extending from the contact hole 42 beyond the selection lineWCS·X_(n). Into a region 49 between the column lines C₀₂ and C₁₂ and theimpurity diffusion region at the lower end of the selection lineWCS·X_(n) extending from the contact hole 42 are implanted channelformation inhibiting impurities with the use of a mask pattern 43 shownby a broken line in FIG. 7. Thus the selection transistor for the columnlines C₀₁ and C₁₁ is the MOS transistor formed in the selection linesWSC·X_(n), whereas the selection transistor for the column lines C₀₂ andC₁₂ is the MOS transistor formed in the selection lines WCS·X_(n). Sincethe channel direction of these MOS transistors formed in the selectionlines WSC·X_(n) and WCS·X_(n) is the same as the channel direction ofthe memory transistors, channel stop regions may be easily formed in aself-aligned manner as in the case of the word lines W₁ to W₈ to reducethe area occupied by memory cells or to achieve a higher degree ofintegration of the memory device.

The bit lines are also formed so that the channel direction of theselection transistors in the X-direction. Thus the bit lines B₁₂ and B₂₂are formed from the lower end of the word line W₈ as far as the lowerend of the selection line WBS·X_(n), while the terminal end 45 of thesecond selection means does not reach the lower end of the selectionline WBS·X_(n). Thus the perimetral portion of the contact hole 42 maybe extended on the line of extensions of the bit lines B₁₂ and B₂₂. Onthe other hand, the bit lines B₁₁ and B₂₁ are formed in the rangeextending from the lower end of the word line W₈ as far as the lower endof the selection line WBS·X_(n), such that the selection line WBS·X_(n)towards the contact hole 42 may be used as the gate of the selection MOStransistor. The region 60 between the bit lines B₁₁, B₁₂ and theimpurity diffusion region extended from the contact hole 42 contactingto main bit lines B₁, B₂ is formed as a channel stop region with the useof the mask pattern 43. The impurity diffusion region is extended in theY-direction on the lines of extension of the column lines C₀₂ and C₁₂and has its terminal portion 46 lying across the two selection linesWBS·X_(n) and WBS·X_(n). Thus the main bit lines B₁ and B₂ arealternatively connected to the bit lines, such that these main bit linesare connected via selection line WBS·X_(n) to bit lines B₁₁ and B₂₁,while being connected via selection line WBS·X_(n) to bit lines B₁₂ andB₂₂. As in the column lines, the channel direction is the same as thedirection of the word lines W₁ to W₈, which is advantageous for reducingthe area occupied by the memory cells.

The mask pattern 43 may be used simultaneously as mask 44 for programmedion implantation which is designed for inhibiting channel formation ineach memory transistor. This result in a more simplified operatingprocess and reduced turn around time (TAT).

The main bit lines B₁ and B₂ are aluminum wiring layers extended in theY-direction. The main column lines C₀ and C₁ are similarly aluminumwiring layers also extended in the Y-direction. These main bit lines B₁and B₂ and the main column lines C₀ and C₁ are of a striped pattern andextended parallel to each other so as to connect to the impuritydiffusion region formed on the surface of the silicon substrate 41 inthe region of the contact hole 42. With the read-only memory of thepresent embodiment, the contact hole 42 for the main bit lines B₁ and B₂and the contact hole 42 for the main column lines C₀ and C₁ are formedin a staggered relation in the Y-direction of the cell block. Thus thecontact holes 42 are not adjacent to each other in the X-direction,which is advantageous for circuit integration.

Layout for the Case in which the Electrode Layer is formed as twopolysilicon layers (FIG. 8)

Referring now to FIG. 8, the layout for the case in which the electrodelayer is formed as the two polysilicon layers, is explained. Meanwhile,the layout shown in FIG. 8 is a continuous repetition of the patternshown therein in the Y- and X-direction, as in the case of FIG. 7.

The ROM of the present embodiment, having these two polysilicon layersas the electrode layers, is of a striped pattern in which, as shown inFIG. 8, the first polysilicon electrode layer and a second polysiliconelectrode layer are formed on a silicon substrate 51 in the X-direction.The selection lines WBS·X_(n), WCS·X_(n) and WCS·X_(n+1) and the wordlines W₂, W₄, W₆ and W₈ are formed from the first polysilicon layer,while the selection line WBS·X_(n), WCS·X_(n) and WCS·X_(n+1) and theword lines W₁, W₃, W₅ and W₇ are formed from the second polysiliconlayer. The horizontal distance between the first and second polysiliconlayers, functioning as the selection lines, is that formed solely by thethin interlayer insulating film, such that the polysilicon layers arearrayed with an interstice therebetween which is so narrow that theterminal portions thereof in the Y-direction overlap with each other.The word lines W₁ to W₈ is arrayed with a narrow interstice in theY-direction by taking advantage of the properties of the two polysiliconlayers. Ion implantation for transistor programming is performed forthese word lines W₁ to W₈ using mask patterns 541 and 542. This ionimplantation may be performed in self alignment which is effective forreducing the area. The arrangement of the word lines in explainedsubsequently. It is noted that the selection lines may be formed bysingle polysilicon layers arrayed side by side instead of thetwo-layered structure.

Each cell block is arranged in a region defined between contact holes52, 52 as viewed in the Y-direction. Since the two polysilicon layersare used as described above, the cell block width along the Y-directionis shorter than in the case of the single layer construction.

Within the cell block, bit lines B₁₁, B₁₂, B₂₁ and B₂₂ and column linesare formed in striped pattern parallel to one another with theY-direction as the longitudinal direction. The patterns of these bitlines B₁₁, B₁₂, B₂₁ and B₂₂ and the column lines C₀₁, C₀₂, C₁₁ and C₁₂are formed by impurity diffusion regions formed at the lower end of thethick oxide film (LOCOS). This thick oxide film is not shown in thedrawing. The bit lines B₁₁, B₁₂, B₂₁ and B₂₂ and the column lines C₀₁,C₀₂, C₁₁ and C₁₂ function as the source and drain regions of the memorytransistors. The channel direction of the memory transistors of thesebit lines B₁₁, B₁₂, B₂₁ and B₂₂ and the column lines C₀₁, C₀₂, C₁₁ andC₁₂ is the same as that of the MOS transistors functioning as the secondand third selection means.

The column lines C₀₁, C₀₂, C₁₁ and C₁₂ are formed by patterns beginningat the lower end of the word line W₁. The terminal ends 57 of the columnlines C₀₁ and C₁₁ extend to the lower end of the selection lineWCS·X_(n), whereas the column lines C₀₂ and C₁₂ extend to the lowers endof the selection line WCS·X_(n). The impurity diffusion region,connecting to the contact hole 52, is extended over the selection linein the Y-direction on the line of extension of the bit line forfunctioning as one source drain region of the MOS transistor. Thus theselection transistor of the column lines C₀₁, C₁₁ is the MOS transistorformed on the selection line WCS·X_(n), whereas the selection transistorof the column lines C₀₂ and C₁₂ is the MOS transistor formed on theselection line WCS·X_(n). For forming these selection transistors, ionimplantation for inhibiting channel formation is performed into regions591 and 592 using mask patterns 531 and 532. These mask patterns may beformed by the same process as used for programming for the lowerportions of the first and second polysilicon layers. Since the channeldirection of the MOS transistor formed in each of these selection linesWCS·X_(n) and WCS·X_(n) is the same as that of the memory transistor,channel stop regions can be easily formed in a self-alignment manner, asin the case of the word lines W₁ to W₈, with a marked advantage inreduction of the area occupied by the cells or higher integration of thememory device.

While the pattern of the bit lines B₁₁, B₁₂, B₂₂ and B₂₁ starts at thelower end of the word line w₈, the terminal ends of the bit lines B₁₂and B₂₂ are at the lower end of the selection line WBS·X_(n) and the bitlines B₁₁ and B₂₁ are formed as far as the lower end of the selectionline WBS·X_(n). With such pattern, the MOS transistor having the channeldirection in the X-direction is formed in each of the selection linesWBS·X_(n) and WBS·X_(n), in a manner advantageous for reducing theoccupied area. Thus, on the bit line side, the regions 611 and 612between the bit lines B₁₂, B₂₂ and the impurity diffusion regions may beformed as the channel stop regions with the use of the mask patterns 531and 532 which are used during programming.

The main bit lines B₁ and B₂ and the main column lines C₀ and C₁ arealuminum wiring layers extended in the Y-direction in the drawing. Themain bit lines B₁ and B₂ and the main column lines C₀ and C₁ are formedas striped patterns running parallel to each other, in a manneradvantageous for reducing the circuit size. These main bit lines B₁ andB₂ and the main column lines C₀ and C₁ connect to the impurity diffusionregion for formed on the surface of the silicon substrate 41 near thecontact hole 52. With the read-only memory device of the presentembodiment, the contact holes 52 for the main bit lines B₁ and B₂ andthe main column lines C₀ and C₁ are formed in a staggered relation inthe Y-direction of the cell block, so that the contact holes 52 are notarrayed in an adjacent relation in the X-direction with an advantage forcircuit integration.

Cell Structure (FIGS. 9 to 13)

The structure of the memory cell is explained by referring to FIGS. 9 to13.

FIG. 9 is a plan view showing a cell of the ROM of the presentembodiment having the two polysilicon layer structure. The hatched lineregions indicate thick oxide films formed on the surface of a p-typesemiconductor substrate 101. These oxide films are each of a stripedpattern and are extended in the Y-direction parallel to each other.Source-drain regions 107 are formed in an aligned manner below the thickoxide film 102. First polysilicon layers 103 as the first electrodelayers and second polysilicon layers 104 as the second electrode layersare formed as parallel striped pattern extending in the X-directionwhich runs at right angles with the thick oxide films 102. The firstpolysilicon layers 103 are formed by parallel stripped patterns and havean interval of a width l₁ between the adjacent patterns. The secondpolysilicon layers 104 are formed to overlie the edges of the firstpolysilicon layers 103 with the edges of the second layers 104 in theY-direction overlapping with the edges of the first layers 103. Thus thememory transistors are formed in a side-by-side relation in theY-direction without any interstices therebetween for realizing thehigher degree of integration of the read-only memory device. Asubstantially square pattern 105 represents a window of a program maskby ion implantation into the lower portion of the first polysiliconlayer 103, whereas a substantially square pattern 106 represents awindow of a program mask by ion implantation into the lower portion ofthe second polysilicon layer 104. These patterns 105 and 106 largeropenings, being larger in width than the layers 103, 104 in theY-direction and bridging a pair of the thick oxide film 102, 102 in theX-direction. During the ion implantation with the aid of the pattern105, a pair of the thick oxide films 102, 102 function as a part of themask along with a resist mask. Excess portions in the Y-direction may beetched off by aligned etching with the first polysilicon layer to copewith the problem of mask shifting. During ion implantation with the aidof the pattern 106, a pair of the pair of thick oxide films 102, 102 andthe first polysilicon layer 103 function as the mask, along with theresist mask, to cope with the problem of mask shifting. Hence, apositive programming may be achieved despite increased circuitintegration.

FIGS. 10 and 11 are sectional views in the X-direction. FIG. 10 shows asection at the second polysilicon layer wherein thick oxide films 102,102 spaced apart from each other on the surface are formed on thesurface of the p-type silicon substrate 101. On the surface of thesilicon substrate 101 below the oxide films are formed n⁺ type impurityregions 107 in an aligned manner. This n⁺ type impurity region 107functions as the source-drain region of the memory transistor. Thesubstrate surface region which is sandwitched between a thick oxidefilms 102, 102 is recessed by etching to form a groove 109. On thebottom and the lateral sides of the groove 109 is formed a gate oxidefilm 108 thinner in thickness than the oxide film 102. The secondpolysilicon layer 104 is formed continuously when seen in cross-sectionover the gate oxide film 108 and the thick oxide film 102 and over thegate oxide film 108 of the adjacent memory transistor. This polysiliconlayer 104 is formed adjacent to the gate oxide film 108 in a regiondefined between a pair of the thick oxide films 102, 102, and isisolated sufficiently from the n⁺ type impurity region 107 at thesethick oxide films 102, 102. FIG. 11, which is the section of FIG. 9taken in the X-direction, is taken at the first polysilicon layer 103.In the section of FIG. 11, thick oxide films 102 are formed in a spacedapart relation on the silicon substrate 101, and an n⁺ type region 107is formed in an aligned manner below the oxide films. Thus n⁺ typeimpurity region 107 functions as the source-drain region of the memorytransistor. However, the silicon substrate 101 is not etched off in aregion between the oxide films 102, and a gate oxide film 108 is formeddirectly on the substrate surface. The first polysilicon layer 103 isextended continuously from above the gate oxide film 108 on thesubstrate surface to a region on the oxide film 102 in the direction ofthe section and thence to a region above the gate oxide film 108 of theadjacent memory transistor.

FIGS. 12 and 13 are sectional views of FIG. 9 taken in the Y-direction.FIG. 12 is a sectional view taken at the thick oxide film 102. A thickoxide film 102 is formed on the surface of a p-type silicon substrate101 along a linear n⁺ type impurity region 107. On the oxide film 102,first polysilicon layers 103 and second polysilicon layers 104 areformed alternately. The edges of the second polysilicon layers 104overlap with the edges of the first polysilicon layers 103 with theinterposition of an interlayer insulating film, not shown. In FIG. 13,which is a sectional view taken at the channel-forming regions of thememory transistors, the region of the silicon substrate 101 in registerwith the second polysilicon layer, 104 is etched off, and the secondpolysilicon layer 104 is formed on a so-formed grove 109 with theinterposition of a gate oxide film 108. The first polysilicon layer 103is formed on a gate oxide film 108 formed on the substrate surface. Thememory transistors are formed for each of the polysilicon layers 103,104. Thus the channel-forming region of the substrate surface differs inheight between the memory transistors which are adjacent to each otherin the sectional direction. Into these channel-forming regions, thereare selectively introduced p-type impurities to form impurity regions110, 111, as shown in FIG. 12. The memory transistors in thechannel-forming regions of which these impurity regions 110, 111 areformed, are not turned on even when selected by the word line potentialbeing raised, so that a pair of the n⁺ type impurity regions 107, 107functioning as the source-drain region are not rendered electricallyconductive. On the other hand, the memory transistor not provided withthe p-type impurity region, is rendered electrically conductive across apair of n⁺ type impurity regions 107, 107 functioning as thesource-drain region. Thus the programmed data may be read out on thebasis of such difference in operation.

With the above described read-only memory device of the presentembodiment, the p-type impurity region 107 functioning as thesource-drain region is formed below the thick oxide film 102 to enable ahigh degree of integration to increase the ROM capacity. Since thememory cell structure is of the NOR type, the memory transistor arearranged side by side between the common source and the common drain.Thus the memory cell driving capacity is not changed with the number ofthe transistors and positive and fast data read-out may be enabled witha sufficient driving capability. In addition, with the read-only memorydevice of the present embodiment, the electrode layers are formed by twopolysilicon layers 103 and 104, and the second polysilicon layer 104 isformed in a region defined between the first polysilicon layers 103parallel thereto, so that the memory transistors may be arrayed withoutinterstices along the longitudinal direction of the thick oxide film 102in a manner advantageous for higher integration. Above all, programmingmay be performed more reliably by providing a step difference betweenthe lower end of the first polysilicon layer 103 and the lower end ofthe second polysilicon layer 104.

Process of Forming Source-Drain Region (FIGS. 14a to 14c)

Referring to FIGS. 14a to 14c, the method of forming a p-type impurityregion 123 functioning as the source-drain region below a thick oxidefilm 124 is explained by referring to FIGS. 14a to 14c.

First of all, an oxidation resistant layer 121 formed by a siliconnitride film is formed on a p-type substrate 120 by means of a pad oxidefilm. A resist layer 122 is applied on the oxidation resistant layer121. The resist layer 122 is exposed to a pattern for formation of athick oxide film and developed. This pattern is comprised of a parallelarray of band-shaped openings formed in the memory cell array region.With the use of the thus patterned resist layer 122, the oxidationresistant film 121 is patterned such as with the use of the RIE method.Then, as shown in FIG. 14a, n-type impurities, such as arsenic ions, areimplanted to a high concentration, using the resist layers 122 and theoxidation resistant film 121 as the masks. By such ion implantation,n-type impurity regions 123 with the pattern of parallel stripes areformed on the surface of the silicon substrate 120. These n-typeimpurity regions 123 are formed in the same way as when forming channelstop regions in the lower portion of the usual field oxide films.

The resist layer 122 is removed by ashing, for example, and oxidized inits entirety. By this oxidation, a thick oxide film (LOCOS) 124 isformed on the surface of a region free of the oxidation resistant film121, that is a region on which the n-type impurity region 123 is formed,as shown in FIG. 14b. In this manner by forming the thick oxide film124, using the oxidation resistant film 121 as the mask, a thick oxidefilm 124 aligned with the n-type impurity region 123 may be produced.

The oxidation resistant film 121 is then removed and the region on whichthe oxidation resistant film 121 has been formed is oxidized to form agate oxide film 125 as shown in FIG. 14c. The gate oxide film 125 has athinner thickness than that of the oxide film 124.

Formation of electrodes or implantation of impurities for programming isthen performed.

Program and Electrode Layer Forming Process (FIGS. 15a to 15c)

Referring to FIGS. 15a to 15c, the process of forming the electrodelayers and selective impurity implantation for programming is explained.

Referring first to FIG. 15a, impurity ions are selectively implantedinto the lower portion of a gate oxide film 131 of a silicon substrate130. A mask 132 is used for the ion implantation and impurities areimplanted into a exposed substrate surface portion in register with anopening 134 formed in the mask 132. The implanted impurities may forexample be p-type impurities, such as boron, and the opening 134 in themask 132 is wider than the channel-forming region for the memorytransistor. It is because the thick oxide film functions as a part ofthe mask, as described previously. Also, since the substrate surfaceregion outside the first polysilicon layer is etched off, as will beexplained subsequently, ion implantation may be performed over a wideregion without raising any problem. The mask 132 is formed by, forexample, a resist layer. The region 133 into which the impurities areimplanted in this manner prove to be a channel-forming region of thetransistor having a high threshold voltage.

The mask 132 is then removed and first polysilicon layers 135 are formedon the overall surface of the gate oxide film 131. These firstpolysilicon layers 135 are formed as parallel stripes extending in adirection perpendicular to the drawing sheet. After patterning the firstpolysilicon layers 135, the gate oxide film 131 in the region betweenthe first polysilicon layers 135 is removed, portions of the siliconsubstrate 130 thus exposed are etched off from above to form grooves 136aligned with the first polysilicon layers 135. During this etching, theends of the impurity region 133 formed to a wider width as describedabove are removed. With the ends of the impurity regions 133 thus etchedoff, it may be ensured that only the lower portion of the firstpolysilicon layer 135 is programmed.

After the groove 136 has been formed in alignment with the firstpolysilicon layer 135, a mask 137 having an opening 138 adapted forselective implantation of impurities is formed, as shown in FIG. 15b.This opening 138 is a window selectively formed in the region where thesecond polysilicon layer is to be formed, and is of an size larger thana region below the second polysilicon layer 136 where the channel regionis to be formed. It is because the previously formed first polysiliconlayer 135 and the thick oxide film function as a part of the mask. Bysuch aligned programming, sufficient data writing may be assured evenwhen the degree of circuit integration is to be raised. Using this mask137, p-type impurities, such as boron, are implanted for selectiveimpurity ion implantation into the grooves 136. Similarly to the region133, the region 139 into which the impurities have been implanted inthis manner is employed as the region for forming the transistor channelpresenting a high threshold voltage.

The mask 137 is then removed and an interlayer oxide film and a gateoxide film 140 are formed by thermal oxidation. The interlayer oxidefilm covers the surface of the first polysilicon layer 135. The gateoxide film 140 is formed by oxiding the side wall and bottom surfaces ofthe groove 136. After formation of the interlayer oxide film and thegate oxide film 140, a second polysilicon layer 141 is formed on theoverall surface such as by the CVD method. The second polysilicon layer141 is formed along the side wall and bottom surfaces of the groove 136.After formation of the second polysilicon layer 141 on the overallsurface, the second polysilicon layer 141 is subjected to patterning.This patterning is performed in such a manner that the secondpolysilicon layer 141 is formed into parallel stripes such that thesecond polysilicon layers 141 in the stripe form overlie the grooves 136defined between the first polysilicon layers 135 so that the edges ofthe second polysilicon layers 141 overlap with the edges of the firstpolysilicon layers 135 with the interposition of interlayer oxide films.After formation of the second polysilicon layers 141, a silicon oxidefilm (e.g. PSG) 142 as the interlayer insulating film is formed. On thetop of the silicon oxide film 142 is formed an aluminum wiring layer 143in accordance with any desired pattern as shown in FIG. 15c. Thisaluminum wiring layer 143 functions as main bit lines or main columnlines connected to n⁺ type impurity region below the thick oxide film.Passivation films may be formed in accordance with the usual process tocomplete the read-only memory device.

With the above described method for producing the read-only memory,since the impurity region 123 functioning as the source-drain region isformed below the thick oxide film 124, the source-drain region is noteffected even when impurities for programming are introduced inalignment with the polysilicon layer. Since the two polysilicon layersare formed and the grooves 136 are formed in alignment with the firstpolysilicon layers 136, the openings 134, 138 of the masks 132, 137 forintroducing impurities for programming may be of a wider size so thatmask mismatching may be successfully coped with. A high density memorycell arraying may be assured by arranging the first polysilicon layer135 and the second polysilicon layer 141 in a side-by-side relation andin sufficient proximity to each other with the interposition of thininterlayer oxidized films.

The grooves 136 below the second polysilicon layers are notindispensable as long as the problem of program mask matching may besolved. The electrode layer need not be a polysilicon layer and may bereplaced by a refractory metal silicide or polysilicide or a refractorymetal layer. The insulating film material need not be an oxide film butmay also be a combination of the oxide film and the nitride film.

Layout of Column Selection Circuit (FIG. 19)

FIG. 19 shows a layout of a portion of a column selection circuit. Inthe drawing, the region shown by numerous dots represent a polysiliconlayer and represents signal lines 71 to which signals Y₀ to Y₆ fromcolumn decoder are supplied. These signal lines are extended as band inthe Y-direction and arranged with an interval l₂ in the X-direction.Since the main bit line and the main column line are divided into twobit lines and two column lines respectively, the interval l₂ correspondsto twice the memory cell pitch.

The main column lines C₁, C₂ and C₃ and the main bit lines B₁, B₂ and B₃are alternately formed in the Y-direction in band-like pattern as shownby hatched lines in FIG. 19. These main column lines C₁ to C₃ and themain bit lines B₁ to B₃ are formed by aluminum wire layers.

A ground line 72 formed by a diffusion region is formed on the siliconsubstrate 70. This ground line 72 is extended in the Y-direction belowthe main bit lines B₁ to B₃ and has an extended region 73 which provesto be one source/drain region of a selection transistor Tc. That is, theselection transistor Tc has its gate electrode formed by the signal line71 and its the other drain/source region formed by a region 74. Theregion 74 connects to the main column lines C₁, C₂ and C₃ via contacthole 75 formed on the surface. Thus the regions 74 and 73 are renderedelectrically conductive by the potential of the signal line 71, whilethe potential of the main column lines C₁, C₂ and C₃ is selectively setto the grounding potential. One extended region 73 proves to be thecommon source-drain of two selection transistors Tc. The lower region 74of one contact hole 75 proves to be the common source-drain of twoselection transistors Tc. This results in an improved circuitintegration.

A data bus line 76 formed by an Al wiring layer is formed in a patternof extending in the X-direction. This data bus line 76 is electricallyconnected to a diffusion region 78 via contact hole 79. This diffusionregion 78 is opposite to a diffusion region 79 with the interposition ofthe signal line 71. These diffusion regions 78 and 79 function as thesource-drain region of selection transistor T_(B) having the signal line71 as its gate electrode. Each diffusion region 79 is electricallyconnected to the main bit lines B₁, B₂ and B₃ by means of a contact hole80. Thus the potentials or data appearing on the main bit lines B₁, B₂and B₃ are such that only the column of the selected signal line 71 iselectrically connected to the data bus line 76 and, are outputted afteramplification by the associated sense amplifier.

Second Embodiment

The present embodiment is modified from the ROM of the first embodimentwith respect to, above all, the circuit construction of the columnselection circuit. Since the circuit construction is otherwise the sameas the first embodiment, only the construction of the column selectioncircuit is hereinafter explained with reference to FIG. 16.

The column selection circuit 2a is used for electrically connecting thedata bus line and the ground line selectively to the main bit lines B₁to B₃ or the main column lines of the memory cell block 1, and iscontrolled on the basis of the signal Y₁ and Y₂.

The main column lines C₁ and C₂ of the column selection circuit 2a ofthe present embodiment are directly controlled on the basis of thesignals Y₁ and Y₂. That is, the main column line C₁ is connected to theground line by means of a MOS transistor T₃₅, to the gate of which thesignal Y₁ is supplied. The main column line C₂ is connected to theground line by means of a MOS transistor T₃₈, to the gate of which thesignal Y₂ is supplied.

The main bit lines B₁, B₂ and B₃, controlled by the signals Y₁ and Y₂,need to be selected in accordance with the selected bit line in the samegroup. To this end, MOS transistors are provided which operate on thebasis of signals over selection lines WBS, WCS, WBS and WCS. Thus themain bit line B₁ is electrically connected to the data bus line by meansof serially connected MOS transistors T₃₄ and T₃₀. The main bit line B₂is connected to be data bus line by a first channel including seriallyconnected MOS transistors T₃₆ and T₃₂ and a second channel includingserially connected MOS transistors T₃₇ and T₃₁. The main bit line B₃ iselectrically connected to the data bus line by means of seriallyconnected MOS transistors T₃₃ and T₃₉. Meanwhile, although the main bitlines B₁ and B₃ may also have two channels, they are not illustrated forsimplicity. The MOS transistor T₃₀, T₃₁ are turned on by the logicalproduct of the signals on selection lines WBS and WCS, while the MOStransistor T₃₂, T₃₃ are turned on by the logical sum of the signals onselection lines WBS and WCS. Thus the MOS transistors T₃₀, T₃₂ are notturned on simultaneously, while the MOS transistor T₃₁, T₃₃ are notturned on simultaneously. Therefore, when a group has been selected bythe signal Y₁ and Y₂, the main bit lines B₁ to B₃ are selectedalternatively and further the operation of selecting the bit and columnlines is performed for reading out data from one cell.

With the column selection circuit 2a of the present embodiment, thenumber of the transistors is significantly reduced as compared to thecolumn selection circuit 2 of the preceding first embodiment, so thatreduction in the area occupied by column selection circuit 2a may beachieved easily.

Third Embodiment

The present embodiment is modified from the ROM of the preceding firstembodiment with respect to the circuit construction of the load circuitand the column selection circuit. Since the circuit construction isotherwise the same as the preceding first embodiment, only the differentcircuit portions will be explained with reference to FIGS. 17 and 18.

As shown in FIG. 17, the ROM of the present embodiment has the memorycell block 1 similar to that of the first embodiment. This memory cellblock 1 has the main bit lines B₁, B₂ and B₃ and main column lines C₁and C₂ extending in a direction perpendicular to the word linedirection. Two bit lines and two column lines are divided out from eachof these main bit lines and main column lines as in the case of thefirst embodiment.

A load circuit 3a is provided at a terminal end of the memory cell blockin the extending direction of the bit lines. This load circuit 3a hasprecharging MOS transistors T₄₁, T₄₃, T₄₅, T₄₇ and T₄₉, in addition toMOS transistors T₄₀, T₄₂, T₄₄, T₄₆ and T₄₈ providing predeterminedimpedances to the main bit lines B₁ to B₃ and main column lines C₁ andC₂. One source-drain regions of the MOS transistors T₄₀, T₄₂, T₄₄, T₄₆and T₄₈ are connected to a power source line, while the othersource-drain regions thereof are connected to the main bit lines or maincolumn lines. A signal φ₁ is supplied to the gates of these MOStransistors T₄₀, T₄₂ T₄₄, T₄₆ and T₄₈. The one source-drain regions ofthe MOS transistors T₄₁, T₄₃, T₄₅, T₄₇ and T₄₉ are connected to the mainbit or column lines and the other source-drain regions thereof aresimilarly connected to the power source line. The signal φ₂ is suppliedto the gate of these MOS transistors T₄₁, T₄₃, T₄₇ and T₄₉. The signalφ₂ is raised to "H" level when the selected memory transistor isswitched, as will be explained subsequently. Precharging is performedwhen the signal φ₂ is raised to the "H" level.

A column selection circuit 2b is provided at the other terminal end ofthe memory cell block 1 in the extending direction of the bit lines.With the column selection circuit 2b, having a similar construction tothat of the column selection circuit 2 of the first embodiment MOStransistors T₅₀, T₅₁, T₅₂ and T₅₃ functioning as switches, are formedbetween the MOS transistors T₁₁, T₁₂, T₁₅ and T₁₆ and the ground line.Each of these MOS transistors T₅₀, T₁₅, T₅₂ and T₅₃ has its gatesupplied with the signal φ₂ and is turned on or off when the signal φ₂is at the "H" level or at the "L" level, respectively. With the MOStransistors T₅₀, T₅₁, T₅₂ and T₅₃ turned off, the main column lines C₁and C₂ disconnected from the ground level to enable effectiveprecharging.

Referring now to FIG. 18, the operation of the ROM of the thirdembodiment is briefly explained. Since the operation of the presentembodiment is the operation of the first embodiment combined with theprecharging operation, the operation of precharging is explainedbriefly.

At the start of a given cycle, the signal φ₂ rises from the "L" level tothe "H" level, as a result of which the MOS transistors T₄₁, T₄₃, T₄₅,T₄₇ and T₄₉ of the load circuit 3a are turned on. On the other hand,with the falling of the signal φ₂, the MOS transistors T₅₀, T₅₁, T₅₂ andT₅₃ of the column selection circuit 2b are turned off, as a result ofwhich the potential of the main column lines C₁ and C₂ and the main bitlines B₁ to B₃ is raised to the level of the source voltage Vcc.

After such precharging, and before start of the driving of the memorytransistor of the memory cell block 1, the signals φ₂ and φ₂ are broughtto the "L" level and "H" level, respectively. Thus the MOS transistorsT₄₁, T₄₃, T₄₅, T₄₇ and T₄₉ are turned off and the MOS transistors T₅₀,T₅₁, T₅₂ and T₅₃ are turned on. This enables data to be read out so thatthe read-out operation is performed similarly to the first embodiment.

With the above described ROM of the third embodiment, high-speed dataread-out may be enabled by precharging of the main bit lines and maincolumn lines.

Although two bit lines and two column lines are associated with eachmain bit line and main column line, respectively, in the aboveembodiments, the present invention is not limited thereto but may beadapted to various other modifications.

What is claimed is:
 1. A method for producing a read-only memory devicehaving a memory cell array, said memory cell array having a plurality ofMISFET's connected in parallel each other, each memory cell having achannel region adjacent to other channel regions of adjacent memorycells, comprising the steps of:forming an oxidation resistant film onthe surface of a semiconductor substrate of a first conductivity type;defining photo resist films having a pattern of parallel stripes on thesurface of said oxidation resistant film; forming parallel stripes ofoxidation resistant films on the surface of said semiconductor substrateby etching said oxidation resistant film with said photo resist films asmasks; introducing impurities of a second conductivity type opposite tosaid first conductivity type into the substrate surface in alignmentwith said photo resist films and said oxidation resistant films;removing said photo resist films from said semiconductor surface;oxidizing the substrate surface, with said oxidation resistant film asthe mask, to form first oxidized films; removing said oxidationresistant films from said semiconductor surface; forming second oxidizedfilms thinner than said first oxidized films in regions on saidsubstrate surface other than first oxidized films; selectivelyintroducing impurities of a first conductivity type into substrateregions where first electrode layers are to be formed; forming aplurality of first electrode layers on said second oxidized films in apattern of plural stripes extending parallel to one another in adirection substantially orthogonal to said first oxidized films; etchingsaid second oxidized films and said substrate in alignment with saidfirst electrode layers; selectively introducing impurities of a firstconductivity type into a surface region of the etched substrate; andforming second electrode layers in a pattern of stripes extendingparallel to said first electrode layers on etched substrate surfaceregions and the first electrode layers with the interposition of secondoxidized films.